1. Field of the Invention
The present invention pertains to a patient monitor for monitoring and/or quantitatively measuring a physiological characteristic of the patient, and, in particular, to an apparatus and method for monitoring and/or quantitatively measuring a physiological characteristic based, at least in part, on a pressure differential between a pressure within a user interface and an ambient atmospheric pressure outside the user interface.
2. Description of the Related Art
There are many situations in which it is necessary or desirable to measure a physiological characteristic of a patient, such as characteristics associated with respiration. Examples of characteristics associated with respiration include the patient's flow, inspiratory period, expiratory period, tidal volume, inspiratory volume, expiratory volume, minute ventilation, respiratory rate, ventilatory period, and inspiration to expiration (I to E) ratio. It is also important in many situations to identify still other characteristics associated with respiration, such as identifying the start, end and duration of a patient's inspiratory phase and expiratory phase, as well as detecting patient snoring. For example, when conducting a sleep study to diagnose sleep disorders or when conducting other pulmonary monitoring functions, it is common to measure the respiratory rate and/or the air flow to and from the patient. Distinguishing between inspiration and expiration is useful, for example, in triggering a pressure support device that provides breathing gas to a patient.
There are several known techniques for monitoring patient breathing for these purposes. A first conventional technique involves placing a thermistor or thermocouple in or near the patient's airway so that the patient's breath passes over the temperature sensing device. Breathing gas entering the patient has a temperature that is generally lower than the exhaled gas. The thermistor senses this temperature difference and outputs a signal that can be used to distinguish between inspiration and expiration.
A primary disadvantage of the thermistor or thermocouple air flow sensing technique is that these devices cannot quantitatively measure the flow and/or volume of breathing gas delivered to and/or exhaled from the patient, because the signal from the sensor is a measure of air temperature, not air flow or pressure. Typically, a thermistor air flow sensor is only used to differentiate between inspiration and expiration. Sensors that detect humidity have similar uses and similar disadvantages.
A second conventional technique for measuring the airflow to and from a patient is illustrated in FIG. 1 and involves placing a pneumotach sensor 30 in a breathing circuit 31 between a supply of breathing gas, such as a ventilator or pressure support device, and the patient's airway. In a conventional pneumotach, the entire flow of breathing gas Q.sub.IN is provided to a patient 32 from a pressure source 34. Conversely, all of the gas expelled from patient 32, passes through pneumotach 30 so that during operation, there is a two-way flow of gas through pneumotach 30.
In its simplest form shown in FIG. 1, pneumotach 30 includes a flow element 36 having an orifice 38 of a known size defined therein. Flow element 36 provides a known resistance R to flow through the pneumotach so that a pressure differential .DELTA.P exists across of flow element 36. More specifically, flow element 36 causes a first pressure P1 on a first side of the flow element to be different than a second pressure P2 on a second side of the flow element opposite the first side. Whether P1 is greater than P2 or vice versa depends on the direction of flow through the pneumotach.
In a first type of conventional pneumotach, a major portion Q.sub.1 of the total flow Q.sub.IN of gas delivered to pneumotach 30 passes through orifice 38. The pressure differential .DELTA.P created by flow element 36 causes a lesser portion Q.sub.2 of the gas delivered to the pneumotach to be diverted through a bypass channel 40, which is connected to breathing circuit 31 across flow element 36. An airflow sensor 42 in bypass channel 40 measures the flow of gas therethrough. Because the area of orifice 38 and the area of bypass channel 40 are known and fixed relative to one another, the amount of gas Q.sub.2 flowing through bypass channel 40 is a known fraction of the total gas flow Q.sub.IN delivered to pneumotach 30. Airflow sensor 42 quantitatively measures the amount of gas Q.sub.2 passing through bypass channel 40. Once this quantity is known, the total flow Q.sub.IN of gas passing through pneumotach 30 can be determined.
In a second type of conventional pneumotach, a pressure sensor, rather than an airflow sensor, is provided in bypass channel 40. Gas does not pass through the pressure sensor. Instead, each side of a diaphragm in the pressure sensor communicates with respective pressures P1 and P2 on either side of flow element 36. The pressure sensor measures pressure differential .DELTA.P across flow element 36. For example, for flow in the direction illustrated in FIG. 1, pressure differential .DELTA.P across flow element 36 is P1-P2. Once pressure differential .DELTA.P is known, the flow rate Q.sub.IN of gas passing through pneumotach 30 can be determined using the equation, .DELTA.P=RQ.sup.2, where R is the known resistance of flow element 36.
Another conventional pneumotach 44 is shown in FIG. 2. Pneumotach 44 improves upon pneumotach 30 in FIG. 1 by providing a first linear flow element 46 in place of flow element 36. First linear flow element 46 functions in the same manner as flow element 36 by creating a pressure differential in breathing circuit 31. However, flow element 46 has a plurality of honey-comb like channels that extend in the direction of gas flow to linearize the flow of gas through the pneumotach. The previous flow element 36 in FIG. 1 can create downstream turbulence that hinders the flow of gas through the bypass channel or causes fluctuations in the downstream pressure, thereby degrading the airflow or pressure differential signal output by sensor 42. Flow element 46 solves this problem by providing a plurality of honeycomb-like channels having longitudinal axis parallel to the axis of the breathing circuit. The honeycomb channels ensure that the flow across the downstream port of the bypass channel is linear, i.e., non-turbulent.
To ensure that the flow of gas across the port in bypass channel 40 upstream of flow element 46 is also linear, i.e., non-turbulent, other linear flow elements 48 and 50 are provided in the breathing circuit. Flow elements 48 and 50 have the same honeycomb configuration as flow element 46. Because gas can flow in both directions through pneumotach 44, flow elements 48 and 50 are respectively located on each side of flow element 46 so that each entry port for bypass channel 40 is downstream of one of these additional flow elements regardless of the direction of flow through the pneumotach.
Although a pneumotach improves upon a theremistor in that it quantatively measures the flow and/or volume of gas passing therethrough, it also has significant disadvantages. For example, a pneumotach is relatively complicated and therefore difficult and costly to manufacture. It is also difficult to clean and is relatively large. Because of its size, which is dictated by the need to measure the pressure differential or flow across the flow element in the breathing circuit, it creates a relatively large amount of dead space in the patient breathing circuit, which is not conducive to minimizing rebreathing of CO.sub.2. Because of its complexity, a pneumotach may leak, and its operating capabilities can suffer as a result of heat and moisture buildup.
A third type of conventional airflow meter, illustrated in FIG. 3, is a nasal cannula airflow meter 52. Nasal cannula airflow meter 52 is similar to a nasal oxygen cannula in that it includes a pair of ports 54 and 56 that insert into nares 58 and 60 of the user. A hollow tubing 62 carries a fraction of the total amount of breathing gas to a sensor, such as an airflow or pressure sensor. If the total area of the user's nares relative to the total area of the ports 54 and 56 is known, the nasal cannula airflow meter can provide a quantitative measure of the patient airflow.
However, because the total area of each user's nares can vary from person to person, a commonly sized nasal cannula airflow meter cannot provide an accurate, quantitative measure of the airflow for all users. If two people have different sized nasal openings, the fraction of the exhaled air that is being delivered to the ports of the nasal cannula cannot be known for both users. For example, a first user may deliver 30% of the exhaled gas to the ports of the nasal cannula, while a second user may deliver only 10% of the exhaled to the same sized nasal cannula. This variation in the percentage of gas delivered to the same size cannula is due to the variation in the total cross-sectional area of the nares of both users. For the same size nasal cannula, a user with larger nares will deliver a smaller percentage of the total exhaled gas to the ports of the nasal cannula than a user with smaller nares. Thus, a conventional nasal cannula cannot accurately measure the airflow for a plurality of users having different sized nares.
In addition to detecting and measuring quantities associated with the rate of volume of air being delivered to a patient, there are also many instances where it is important to detect other characteristics associated with respiration, such as snoring. The onset of snoring and/or the intensity of snoring can be used, for example, as a trigger to initiate or control the level of a positive pressure therapy provided the patient. Also, the presence, intensity and/or duration of snoring can be used as a diagnostic tool in determining whether the patient suffers from a sleep and/or breathing disorder.
It is known to use a microphone or pressure sensor mounted on the exterior of the patient's neck to detect sounds or throat vibrations generated by the snore. In many situations, these sensors are mounted on the user as an individual unit and are not connected to other structures worn by the patient. This can result in incorrect or inefficient placement of such sensors. Also, conventional snore sensing devices are quite susceptible to noise. For example, microphones can pick up external sounds not produced by the patient, such as snoring of a person or animal near the patient, and/or sounds not resulting from snoring, such as coughing. Pressure sensors can be adversely effected by body movements, such as normal movements that take place during the night and/or throat vibrations resulting from coughing.